This invention relates to a wireless communication set with a backlighted display and, in particular, to a driver circuit for a backlight source in such a wireless communication set.
A wireless communication set such as a wireless telephone set is known in the art and has a transmitter including a power amplifier. In a certain communication network, for example, a digital communication network such as a TDMA (Time Division Multiple Access) network system, the power amplifier is repeatedly turned on and off in a time division fashion to send out an information signal as a burst signal in a time slot allotted thereto. That is, the power amplifier is cyclically and alternatingly brought into an ON condition and an OFF condition. Thus, repetition of increase and decrease is caused in an electric current used in the power amplifier. This results in undesired variation of a power supply voltage for various other circuit portions in the wireless telephone set.
This problem is severe in the wireless telephone set of a battery-powered portable type having an electric cell or battery.
For example, in a cellular phone for use in the Cellular Communications System, an output power for transmission is usually in a range of several tens of milliwatts to several watts, and an electric current of several tens of milliamperes to several hundred milliamperes is consumed thereby in the power amplifier. This results in a voltage drop due to the presence of internal resistance in the electric cell and/or impedance in a printed circuit board used therein. The voltage drop is varied according to the ON/OFF control of the power amplifier to thereby cause variation of the power supply voltage.
In addition, the wireless communication set of a type described above often has a backlighted display for displaying information such as a telephone number, a time, etc., where the display screen is lighted behind by a backlight source.
For the display, there is known a liquid crystal display (LCD), and the display is backlighted by a light emission diode or by an electo-luminescent (EL) device as the backlight source.
The wireless communication set having the backlighted display has a problem that a relatively large electric current is consumed for backlighting the display. Thus, the power supply voltage is lowered by the backlighting in addition to the power supply voltage variation due to ON/OFF operation of the power amplifier. This accelerates the reduction of the life time of the electric cell.